GraphViz is not really used in widgets (there is only one widget, on attribute interactions, that uses it). To our knowledge there is no general-purpose graph drawing library for Qt (where graph drawing deals with objects such as trees, hierarchies, ...) -- if anybody is aware of such library, let us know!

GraphViz is instead implicitly used in some libraries -- orngTree, for instance, can output trees in dot format, and files need to be parsed to obtain the graphics (in GIF, PS, ...). The same code is used in classification tree browsing widget, but again just saves the tree to the .dot file.